Conventional tube fittings for high-pressure fluid are shown in FIG. 10. In this figure, an end of a pipe 1 forming a route for high-pressure fluid is provided with a tapered face 1a, and an inverse male screw 1b follows the tapered face 1a. Then, a box nut 2 is inserted from the end of the pipe 1.
FIGS. 11(a) and 11(b) show the box nut 2, in which FIG. 11(a) is a cross-sectional view taken along the line XI—XI of FIG. 11(b), and FIG. 11(b) is a side view. As illustrated in these figures, the box nut 2 is provided with a hole with a step, and the diameter of a larger hole 2a on the end side is larger than the diameter of a smaller hole 2b on the rear end side, which has a diameter that the pipe 1 passes with a slight clearance. The outside of the box nut 2 on the rear end side is a hexagonal fastening portion 2c to be fastened with wrenches or the like. After the box nut 2 is inserted to the pipe 1, then, a bush 3 is screwed to the inverse male screw 1b. 
Reference numeral 4 shows a block. The block 4 is provided with a female screw hole 5. On the bottom portion of the female screw hole 5 is opened a fluid route 6. An inlet portion of the fluid route 6 is a tapered face 6a. 
An end portion of the pipe 1 is inserted into the female screw hole 5, and a male screw on the outer surface of the box nut 2 is screwed to a female screw of the female screw hole 5, which causes the box nut 2 to enter deep into the recess of the female screw hole 5 while rotating. At this moment, the box nut 2 and the bush 3 contact with each other at both ends thereof, so that the friction due to the contact causes the box nut 2 and the bush 3 to rotate together. Because the female screw hole 5 has a regular screw (for example, a right screw) while the bush 3 and the inverse male screw 1b have inverse screws (for example, left screws), the bush 3 moves in a direction opposite to the movement of the box nut 2. As a result, the bush 3 functions as a locking means for the box nut 2, and the pipe 1 proceeds in the female screw hole 5 until the tapered face 1a at an end of the pipe 1 contacts the tapered face 6a at the inlet portion of the fluid route 6. Further, strongly fastening the fastening portion 2c of the box nut 2 with wrenches or the like allows the tapered face 1a to press-contact to the tapered face 6a, which connects the route for fluid on the pipe 1 side and the fluid route 6 on the block 4 side with each other.